A significant portion of pollutant gases emitted by internal combustion engines are produced when the engine is initially started (“cold-start”), but before the catalytic converter in the emissions system has warmed up to its operating temperature. In order to reduce harmful emissions during the cold-start phase, such as that of a light-duty diesel vehicle (for example, an automobile or light truck), washcoats that contain zeolites can be used to coat the substrate used in the catalytic converter of the vehicle. These zeolites act as a temporary storage area for the pollutants carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) during the cold-start period, when the catalytic converter is still cold. After the catalytic converter heats up to its operating temperature, known as the light-off temperature, the stored gases are released and subsequently decomposed by the catalytically active material on the substrate.
A high light-off temperature is undesirable, as many vehicular trips are of short duration, and during the time required for the catalytic converter to reach its operating temperature (that is, the light-off temperature), pollutants must either be released untreated to the environment, or stored in the exhaust system until the light-off temperature is reached. Even if pollutants are trapped effectively prior to light-off, the catalytic converter may not reach operating temperature if multiple successive short trips are made, and the zeolites used for storage may become saturated, again resulting in release of pollutants to the environment.
Commercially available catalytic converters use platinum group metal (PGM) catalysts deposited on substrates by wet chemistry methods, such as precipitation of platinum ions and/or palladium ions from solution onto a substrate. These PGM catalysts are a considerable portion of the cost of catalytic converters. Accordingly, any reduction in the amount of PGM catalysts used to produce a catalytic converter is desirable. Commercially available catalytic converters also display a phenomenon known as “aging,” in which they become less effective over time; the light-off temperature starts to rise as the catalytic converter ages, and emission levels also start to rise. Accordingly, reduction of the aging effect is also desirable, in order to prolong the efficacy of the catalytic converter for controlling emissions.